Some electronic circuits, such as Frequency doublers and sub-harmonic VCOs, may generate unwanted frequency harmonics at half integers of the fundamental frequency. These spurious outputs are called sub-harmonics of the fundamental frequency. These unwanted signals are usually filtered with fixed mechanical or electrical circuits and brought down to acceptable levels. Such filtering preferably passes the fundamental frequency (Freq) with slight loss and good return loss while suppressing frequencies at Freq/2 and 3*Freq/2 and higher. If the particular circuit of interest has an operating range that spans near an octave of frequencies, the required filter becomes complex and might need to be tunable. Furthermore, electrical systems that span an octave will generally have amplitude response that falls off with frequency. For such systems a tunable filter that not only rejects frequencies at sub-harmonics but also compensates for the amplitude roll-off is desirable.
U.S. Pat. No. 3,889,214 discloses a tunable stripline combline filter that utilizes discrete manufacturing processes. U.S. Pat. No. 4,835,499 discloses a microstrip combline discrete circuit having less biasing circuitry. These filters are large, expensive, limited in upper frequency range and their resonators would need additional tuning in order to match them to each other due to their inherent mismatch. Also these filters have limited linearity performance.
U.S. Pat. No. 6,525,630 discloses microstrip tunable filters deposited onto a substrate. The filters of the '630 patent, although promising low loss and high Q, are rather expensive, have repeatability challenges, require high operating control voltages and need high isolation on the control lines. The filters also need to employ a pseudo-combline approach, where the microstrip ends opposite of the varactor cannot be grounded, but rather have to be extended in length and left open for DC isolation reasons.
There are applications in which it is desirable to have a bandpass filter which is more selective than the filters described above, and which may also have a tunable response that can reject other interfering signals close to the wanted signal. Examples of such applications are up-conversion mixers where variable LO frequencies may be used and wide band receiver front-ends having interfering frequencies.